Optical transmitters

ABSTRACT

External cavity optical transmitters are disclosed which include a gain chip and a mirror that define an optical cavity. The transmitters further include a modulator operated at or near the same temperature as the gain chip. In some examples, the optical transmitters are temperature controlled to optimize the efficiency and wavelength stability thereof, while maintaining acceptable chirp performance of the modulator. In some examples, the optical transmitters include an electro-optic module disposed within the optical cavity to change the path length thereof so that the efficiency and wavelength stability of the transmitter is optimized.

TECHNICAL FIELD

The present disclosure pertains to optical systems and, moreparticularly, to optical transmitters.

BACKGROUND

Optical systems are widely used in communications applications tofacilitate the exchange of information such as voice and data over fibercable, which may be fabricated from glass or any other suitablecomposite material. Both telephony and Internet-based systems exploitthe wide bandwidth and large data capacity that optical systems provide.Additionally, as compared to conventional wired systems, opticalnetworks are easily maintained and repaired.

Conventional optical systems include a transmitter having a distributedfeedback (DFB) laser that operates at a wavelength at or near one of thewavelengths specified by the International Telecommunications Union(ITU). The DFB laser operates at an ITU specified wavelength within aparticular temperature range. Outside the operating temperature range ofthe DFB laser, the DFB laser becomes detuned and no longer lases at theappropriate wavelength.

The optical transmitter also includes a modulator, such as anelectro-absorption (EA) modulator that imparts information onto theemitted optical energy before the optical energy is coupled to the fiberoptic cable. Like the DFB laser, the EA modulator has an optimaloperating temperature range and wavelength at which the chirp, whichrepresents the maximum distance that information may be transmitted fromthe EA modulator, was optimized.

For acceptable operation of the previously-described opticaltransmitter, the operating temperature of the EA modulator and the DFBlaser must be matched. Failure to match the operating temperatures ofthese components leads either to a transmitter that lases at the properfrequency and has poor chirp performance or to a transmitter that hasacceptable chirp performance, but lases at an incorrect wavelength orthat drifts between wavelengths. As will be readily appreciated by thosehaving ordinary skill in the art, the production yield of opticaltransmitters is very low when having to match the operating temperatureranges of two different components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an example optical transmitter.

FIG. 2 is a flow diagram representing an example routine executed by theprocessing unit of FIG. 1.

FIG. 3 is a diagram representing an example operating current versustemperature response of the optical transmitter of FIG. 1.

FIG. 4 is a diagram of a second example optical transmitter.

FIG. 5 is a flow diagram representing an example routine executed by theprocessing unit of FIG. 4.

FIG. 6 is a diagram representing an example operating current versusvoltage response of the optical transmitter of FIG. 4.

FIG. 7 is a diagram of an example process by which the opticaltransmitters of FIGS. 1 and 4 may be manufactured.

DETAILED DESCRIPTION OF THE PREFERRED EXAMPLES

Although the following discloses example systems including, among othercomponents, software executed on hardware, it should be noted that suchsystems are merely illustrative and should not be considered aslimiting. For example, it is contemplated that any or all of thesehardware and software components could be embodied exclusively indedicated hardware, exclusively in software, exclusively in firmware orin some combination of hardware, firmware and/or software. Accordingly,while the following describes example systems, persons of ordinary skillin the art will readily appreciate that the examples are not the onlyway to implement such systems.

Turning now to FIG. 1, an external cavity optical transmitter (opticaltransmitter) 10 includes a substrate 12 on which a number of componentsare disposed to form an external cavity laser. In particular, a gainchip 14 having first and second reflective coatings 16, 18, a first lens20, a grating 22 and a mirror 24, which may be combined with an etalon26, form a resonant optical cavity. The grating 22, mirror 24, etalon 26and the first lens 20 are mounted in a manner that leaves themrelatively thermally insensitive so that the temperature of the gainchip 14 may be varied without affecting the other components. Theoptical transmitter 10 further includes a second lens 30 disposedbetween the first reflective coating 16 of gain chip 14 and anelectro-absorption chip (EA) 32.

Generally, during operation, when power is applied to the gain chip 14,the gain chip 14 emits optical energy through the second reflectivecoating 18 and the first lens 20 transfers the emitted optical energy tothe grating 22. The grating 22 separates the optical energy into itsconstituent wavelengths and reflects a wavelength of interest to themirror 24. This is represented in FIG. 1 by the ray lines 40. Theoptical energy emitted from the gain chip 14 at the wavelength ofinterest 40 reaches the mirror 24 and is reflected back from the mirror24 to the grating 22 and through the first lens 20 to the gain chip 14.Accordingly, the gain chip 14 and the mirror 24 form an optical resonantcavity in which lasing at the wavelength of interest occurs. Opticalenergy that is not at the wavelength of interest reflects from thegrating 22 to the mirror 24 at an angle such that the mirror 24 will notreflect that optical energy back to the gain chip 14, but insteadreflects the undesired optical energy in a different direction. Thereflection of wavelengths that are not of interest is represented inFIG. 1 by the ray lines 42. The etalon 26 is an additional filter thatdetermines the wavelength selected for lasing. The combined response ofthe gain chip 14, the etalon 26 and the grating 22 determines thewavelength for lasing.

While the gain chip 14, the grating 22 and the mirror 24 form theoptical resonant cavity, the gain chip 14, the second lens 30 and the EA32 form the output of the optical transmitter 10. In particular, duringoperation, optical energy from the gain chip 14, which is a result ofthe lasing between the second reflective coating 18 and the mirror 24,is emitted from the gain chip 14 through the first reflective coating 16and coupled to the EA 32 via the second lens 30. As shown in FIG. 1, theoptical transmitter 10 may include a modulation source 50. The source 50may be separate from or a part of the substrate 12, and provides aninformation signal to the EA 32 that causes the EA 32 to modulate theintensity of the optical energy passed therethrough. For example, themodulation source 50 may operate using pulse width modulation (PWM),on/off keying (OOK) or any other suitable modulation scheme. As will bereadily appreciated by those having ordinary skill in the art, the EA 32is a device having optical absorption characteristics that vary with thevoltage applied to the EA 32. Accordingly, optical energy emitted fromthe gain chip 14 through the first reflective coating 16, which has arelatively constant intensity, may be intensity-modulated by the EA 32in accordance with the signal applied thereto by the modulation source50.

Having described the optical components of the optical transmitter 10,attention is now turned to the electrical components of the opticaltransmitter 10. As shown in FIG. 1, the optical transmitter 10 includesa processing unit 60 having an associated memory 62. A thermistor 64, adetector 66 and a thermo-electric cooler (TEC) controller 68 are eachcoupled to the processing unit 60. The TEC controller 68 is furthercoupled to the TEC 69, only a portion of which is shown as protrudingfrom under the substrate 12. As will be readily appreciated, the TEC 69may underlie a significant portion of the substrate 12 and, inconjunction with the TEC controller 68, maintains the substrate 12 at adesired temperature.

It should be noted that while the processing unit 60 and the memory 62are shown as being separate and distinct components in FIG. 1, thosehaving ordinary skill in the art will readily recognize that such arepresentation is merely one example configuration of a processing unitand its associated memory. For example, the processing unit 60 and thememory 64 could be integrated into one single processing unit includingon-board memory.

Generally, the processing unit 60 receives a signal from the thermistor64 indicative of the temperature of the substrate 12 and receives fromthe detector 66 a signal representative of the optical power output bythe gain chip 14 during lasing. The processing unit 60 then generates anoutput signal that is coupled to the TEC controller 68, which controlsthe TEC 69 to set the temperature of the substrate 12 and the componentsdisposed thereon.

Because the gain chip 14 and the EA 32 are disposed on the samesubstrate 12, the TEC 69 can maintain these components at the same, ornearly the same, temperatures. The TEC 69 may have a temperature controlresolution of 0.1° C. In fact, the temperature range within which thesubstrate 12 is maintained is the temperature range within which the EA32 has optimal or nearly optimal chirp performance, which, as notedpreviously, represents the ability of the EA 32 to transmit opticalinformation great distances throughout an optical network.

The gain chip 14 emits optical energy within a range of wavelengths,wherein one particular wavelength is the dominant wavelength emitted bythe gain chip 14. The combined response of the grating 22 and the etalon26 further filters the output of the gain chip 14 and selects thedesired wavelength for lasing. However, the optical length of the cavityformed by the gain chip 14 and the mirror 24 dictates the opticalwavelength emitted from the gain chip 14 that will be selected forlasing. The temperature at which the gain chip 14 is maintained affectsthe optical length of the cavity and hence controls the lasing mode thatwill be the dominant wavelength. The highest efficiency of the opticaltransmitter 10 is realized when the dominant wavelength selected by theexternal cavity optics is aligned with the external cavity mode.Additionally, when the dominant wavelength of this external cavity laseris aligned with the external cavity mode, mode hopping of the externalcavity laser is eliminated (i.e., the external cavity laser has mode hopfree operation). Accordingly, as described below in detail inconjunction with FIGS. 2 and 3, the temperature of the substrate 12 maybe varied within a range compatible with optimal EA 32 chirp performanceto enable the gain chip 14 to operate at its maximum possible efficiencyand mode hop free for the desired lasing wavelength.

As is well known to those having ordinary skill in the art, mode hoppingis a situation in which multiple cavity modes (determined by the lengthof the external laser cavity) are capable of lasing in a laser and thelaser hops between lasing at each mode and hence exhibits largewavelength instability. A laser operating in a mode hop free state hasbetter wavelength stability and efficiency than the same laser operatingin a mode hopping state.

The efficiency of the optical transmitter 10 and, in particular, theefficiency of the gain chip 14 is measured by comparing the operatingcurrent provided to the gain chip 14 by a current source (not shown) tothe optical power output from the gain chip 14, as measured by thedetector 66. Because, as described in conjunction with FIG. 2, theprocessing unit 60 implements a power control loop that maintains modehop free operation and constant optical power as measured, for example,at the detector 66, the efficiency of the gain chip 14 may be determinedmerely by measuring operating current supplied to the gain chip 14. Bymonitoring the operating current required by the gain chip 14 as afunction of the temperature range in which the substrate 12 operates tomaintain acceptable performance of the EA 32, the processing unit 60determines the temperature at which the substrate 12 should bemaintained for optimal chirp and efficiency performance of the opticaltransmitter 10.

Turning now to FIG. 2, a tune temperature routine 100 for mode hopelimination and operating efficiency executed by the processing unit 60in conjunction with the memory 62 is described. The following describesthe operation of the processing unit 60 in connection with FIG. 3. Whereappropriate, for ease of understanding, the following description refersto the various components and portions of FIGS. 1 and 3 as the operationof the processing unit 60 is described.

When the processing unit 62 begins execution of the routine 100, thetemperature of the substrate 12 is set to a stored value (block 102) viathe TEC controller 68 and the TEC 69. In this example, the storedtemperature is a temperature at which the EA 32 has optimal chirpperformance. After the temperatures of the TEC 69 and the substrate 12are set to the stored value, an operating current supplied to the gainchip 14 is set to a stored value (block 104). The stored value of theoperating current is a starting point at which the processing unit 60enables a power control loop that controls gain chip operating currentto maintain a constant desired output (block 106). The power controlloop operates to increase or decrease the operating current supplied atthe gain chip 14 to maintain the power output from the gain chip 14 at aconstant or relatively constant value.

To this point, the processing unit 60 executing the routine 100 hascaused the gain chip 14 to operate at a steady state that is mode hopfree and at which the output power is constant or nearly constant andthe temperature of the substrate 12 is controlled to be a prestoredvalue. The following describes how the processing unit 60 executes theroutine 100 to find an optimal substrate temperature at which theoperating current supplied to the gain chip 14 is minimal or nearminimal for the desired output power from the gain chip 14 in a mode hopfree operating state. Accordingly, after the power control loop isenabled (block 106), the processing unit 60 commands the TEC controller68 to apply a temperature ramp to the substrate 12 (block 108). In thisexample, the temperature ramp ranges from a temperature that isapproximately one-half of one degree Centigrade below the storedtemperature value to approximately one-half of one degree Centigradeabove the stored temperature value. Alternatively, any other suitabletemperature range may be selected, bearing in mind that a 5-8° C.temperature variation at the gain chip 14 may be sufficient to vary thephase of the optical energy emitted from the gain chip by as much as πradians, at which the gain chip 14 may begin mode hopping and beginlasing at another wavelength. However, it is desirable for thetemperature range to include temperatures over which the performance ofthe EA 32 maintains acceptable chirp characteristics.

As the processing unit 60 and the TEC controller 68 apply thetemperature ramp to the substrate 12, the processing unit 60 monitors(as a function of substrate temperature) the operating current suppliedto the gain chip 14 (block 110). For example, as shown in FIG. 3,operating current may be plotted as a function of temperature as shownat reference numeral 112. As will be readily appreciated by those havingordinary skill in the art upon examining FIG. 3, a plot 112 of operatingcurrent as a function of temperature includes a local minimum specifiedat reference numeral 114. It is at the local minimum 114 that theefficiency of the optical transmitter 10 is optimum because the constantpower out of the gain chip 14 requires the minimal operating current atthe optimum temperature. Additionally, at the local minimum 114, theoptical transmitter 10 operates in a mode hop free manner. An operatingtemperature either lower or higher than the optimum temperature, whichis the temperature at the minimum 114, requires a higher operatingcurrent than the optimum current, which is also denoted at the minimum114, and the likelihood that the optical transmitter 10 will mode hopincreases. Therefore, the gain chip 14 is stable and operating in themost efficient manner at the temperature labeled by reference numeral114.

As shown in FIG. 3, a lock threshold 124 illustrates a temperature rangein which there is less than one percent change between the operatingcurrent and the optimum operating current, which is represented by theminimum point 114. The lock threshold 124 illustrates the operatingpoint at which efficiency of the optical transmitter is good and thereis little chance of a mode hopping occurrence. The application of thetemperature ramp (block 108) is shown at reference numeral 109, whichillustrates a temperature range across which the temperature ramp spans.

After the processing unit 60 determines operating current as a functionof temperature (block 110), the processing unit 60 determines and storesthe minimum operating current (block 116) and determines and stores anoptimum temperature of the substrate 12 corresponding to the minimaloperating current (block 118). The processing unit 60 finds the minimumof the plot 112 by twice differentiating the plot 112 with respect totemperature to determine slope change of the plot, also referred to asconcavity. The processing unit 60 then examines the results of the firstand second differentiation to find a point on the plot 112 having a zeroslope change and a negative slope before and a positive slopethereafter. The point on the plot 112 having zero slope that is boundedby negative and positive slopes is referred to as the optimum operatingpoint at which an optimum or minimum operating current corresponds to anoptimum substrate 12 temperature.

At this point, the processing unit 60 and the TEC controller 68cooperate to keep the temperature of the substrate 12 at approximatelythe optimum temperature that corresponds to the optimum operatingcurrent, thereby causing the optical transmitter 10 to operate at itsmost efficient operating point. Although the optical transmitter 10should continue to operate at the minimal operating current as long asthe substrate 12 is maintained at the optimum temperature correspondingthereto, the processing unit 60 continues to monitor the operatingcurrent supplied to the gain chip 14 (block 120). A one-percent orgreater change in operating current (detected at block 122) will causethe processing unit 60 to apply a temperature ramp to the substrate 12(block 108) and to again seek and store the optimum operating currentand the optimum temperature corresponding thereto. The one-percentchange may be due to aging of the gain chip, movement of opticalcomponents due to aging or the like. However, as long as the processingunit 60 determines that there is less than a one-percent change inoperating current (again, detected at block 122) the processing unit 60continues to measure operating current 120 (block 120).

After the optical transmitter 10 is powered-up and the processing unit60 has executed information or instructions corresponding to blocks102-106 of FIG. 2, the processing unit 60 continues to operate betweenblocks 108 and 122 of FIG. 2. If the optical transmitter 10 were powereddown, the stored TEC temperature and stored operating current recalledat blocks 102 and 104 would typically be the stored operating currentand optimum temperature stored at blocks 116 and 118, which were storedbefore the optical transmitter 10 was powered down.

Turning now to FIG. 4, another example external cavity opticaltransmitter (optical transmitter) 200 is shown. The optical transmitter200 includes a substrate 202, a gain chip 204 having first and secondreflective coatings 206, 208, first and second lenses 210, 212, agrating 214, a mirror 216, an etalon 218, a detector 220 and an EA 222.Each of the components disposed on the substrate 202 may be similar oridentical to the components described in conjunction with FIG. 1.However, the optical transmitter 200 further includes an electro-opticcrystal (EO) 224 disposed between the first lens 210 and the grating214. In operation, as the bias voltage on the EO 224 is changed, therefractive index though the EO 224 changes, thereby changing theeffective optical path length between the gain chip 204 and the mirror216, which, in turn, changes the wavelength at which the gain chip 204lases, thereby controlling the wavelength and efficiency of the opticaltransmitter 200 and minimizing the likelihood of mode hopping operation.

In terms of electrical components associated with the opticaltransmitter 200, many are the same as those described in conjunctionwith the optical transmitter 10 of FIG. 1. These devices include adetector 220, a thermistor 226 and a TEC controller 228, which iscoupled to a TEC 229. The optical transmitter 200 further includes aprocessing unit 230 and an associated memory 232. The processing unit230 is coupled to the detector 220, the thermistor 226, the TECcontroller 228 and is further coupled to a voltage source 232. Thevoltage source 232 is coupled to the EO 224.

Generally, during operation, the processing unit 230 controls thevoltage source 232 to change the voltage bias applied to the EO 224 toalter the refractive index thereof. Accordingly, the effective opticalpath length, between the gain chip 204 and the mirror 216 changes withthe bias voltage applied to the EO 224. While the example opticaltransmitter 10 of FIG. 1 varied temperature of the substrate 12 tocontrol the wavelength at which the gain chip 14 lased, the exampleoptical transmitter 200 of FIG. 4 controls the refractive index of theEO 224 (and, therefore, the effective optical path length between thegain chip 204 and the mirror 216) to affect the wavelength at which thegain chip 204 lases. Further detail on the operational aspects of theoptical transmitter 200 are now provided in conjunction with FIGS. 5 and6.

Turning now to FIG. 5, a tune crystal routine 240 executed by theprocessing unit 230 is represented in block diagram format. Uponpower-up, the processing unit 230 sets the substrate 202, via the TEC229, to a stored temperature value (block 242), which is a value atwhich the EA 222 has optimal or nearly optimal chirp performance. Afterthe TEC temperature is set, the operating current associated with thegain chip 204 is set to a stored value (block 244) and a power controlloop is enabled (block 246). Lice the power control loop described inconjunction with FIG. 2, the power control loop of FIG. 5 keeps theoptical power output of the gain chip 204 at a constant value andchanges the operating current supplied to the gain chip 204 to achievethis goal.

After the power control loop is enabled (block 246), the processing unit230 and the voltage source 232 cooperate to apply a voltage ramp to theEO 224 (block 248). The voltage ramp varies the refractive index of theEO 224, thereby adjusting the path length within the optical cavitydefined between the gain chip 204 and the mirror 216. In this example,the voltage ramp on the EO 224 varies the path length through a range ofone-half wavelength. For example, the voltage applied to the EO 224 mayvary between +/−10 volts (V) and +/−100 V.

As the voltage ramp is applied to the EO 224, the processing unit 230measures operating current supplied to the gain chip 204 as a functionof the voltage applied to the EO 224 (block 250). For example, as shownin FIG. 6, a plot 252 shows operating current versus voltage on the EO224. The line 254 represents an example voltage range that may beapplied to the EO 224 to generate the plot 252.

After the processing unit 230 determines operating current as a functionof the EO voltage, a minimal operating current is determined and stored(block 258) and an optimum voltage on the EO 224 corresponding to theminimal operating current is determined (block 260). As shown in FIG. 6,a point on the plot 252 representing the minimal operating current isdesignated with reference numeral 262. At the point 262, the operatingcurrent is at a minimum and, therefore, the gain chip 204 is operatingat its optimal efficiency given the fixed output power constraintenforced by the power control loop. In this example, the minimumoperating current is determined by taking one or more derivatives of theplot 252 with respect to EO voltage. The minimum point 262 is a pointhaving a second derivative value of zero that is bounded by plotportions having negative and positive first derivatives. The processingunit 230 then commands the voltage source 232 to apply to the EO 224 thevoltage corresponding to the minimal operating current.

Once the minimal operating current, which is also the operating point atwhich the optical transmitter 200 will have mode hop free operation, hasbeen determined, stored and applied (block 260) the operating currentprovided to the gain chip 204 is measured (block 266). The measuredoperating current is monitored for a one-percent or greater changetherein. If a one-percent or greater change in the operating currentexists (block 268), the voltage on the EO 224 is ramped and processingunit 230 repeats the process of determining the minimal operatingcurrent (blocks 248-260). Alternatively, if there is less than aone-percent change in the operating current, the processing unit 230continues to monitor the operating current until such a change occurs.Again, as described in conjunction with FIG. 3, the lock threshold rangeof less than one-percent change in operating current guarantees mode hopfree operation of the external cavity laser. The line 270, shown in FIG.6, represents the voltage range corresponding to a less than one-percentchange in operating current.

Turning now to FIG. 7, an example process by which the opticaltransmitters 10, 200 may be fabricated is shown at reference numeral300. The process begins by performing small signal chirp measurements oneach EA to be used in the manufacturing process to identify an optimumoperational temperature and wavelength for which EA chirp is best (block302). The EAs are then sorted according to their optimal performancetemperatures/wavelengths (block 304). An external cavity laser is thenfabricated with a gain chip that operates at the wavelength andtemperature optimized for one of the EAs (block 306) and the gain chipis placed on a substrate that will be maintained at a temperatureoptimized for the selected EA chirp performance (lock 308). Theremaining optical components of the transmitters 10, 200 are then placedon the substrate with the gain chip (block 310).

After the optical components have been placed on the substrate, thefirst lens is placed on a bipod flexture and is located between the gainchip and the grating in a position in which the x, y, and z-axesposition of the first lens cause the external cavity laser to beginlasing (blocks 312-316). After the lens is located in the x, y, andz-axes, the z-axis location of the first lens is optimized to minimizethe threshold current required for lasing (block 318) and the wavelengthof the laser is tuned by translating the first lens in the x and/ory-axis directions (block 320). After the laser wavelength has beentuned, the z-axis position of the first lens is reoptimized to minimizethe threshold current at the desired wavelength of lasing (block 322)and the first lens is fixed in place on the substrate (block 324).

After the first lens has been fixed in place, a temperature ramp isapplied to the substrate and the threshold current supplied to the gainchip is monitored during the temperature ramp to determine the optimaltemperature for the gain chip operation (block 326). At this point, theEA is placed on the substrate at a designed position in relation to thesecond lens and any other suitable components shown in FIGS. 1 and 4 arealigned and fixed to couple optical energy from the gain chip to the EA(blocks 328-330). An optical fiber is then aligned from the output ofthe EA to couple optical energy therefrom (block 332). The substrate isthen placed on the TEC platform so that the temperature of the substratecan be controlled (block 334) and the hermetic sealing of the opticaltransmitter 10, 200 is then completed (block 336). Finally, theelectronics associated with the optical systems (e.g., the processingunit, memory, the TEC controller and the thermistor) are coupled to theexternal cavity optical transmitter to ensure mode hop free operation ofthe laser (block 338).

Although certain methods and apparatus constructed in accordance withthe teachings of the invention have been described herein, the scope ofcoverage of this patent is not limited thereto. On the contrary, thispatent covers all embodiments of the teachings of the invention fairlyfalling within the scope of the appended claims either literally orunder the doctrine of equivalents.

1. An external cavity optical transmitter comprising: a gain chip toemit optical energy; a grating to receive optical energy emitted by thegain chip and to reflect at least a portion of the optical energyemitted by the gain chip; a reflector to receive optical energyreflected from the grating, the reflector and the reflective portion ofthe gain chip forming an optical resonant structure; an opticalmodulator to modulate optical energy from the gain chip, the opticalmodulator having an optimal operating temperature range; and atemperature controller to maintain the gain chip and the opticalmodulator at a temperature within the optimal operating temperaturerange of the optical modulator.
 2. The external cavity opticaltransmitter as defined in claim 1, wherein the temperature controllermaintains the temperature of the gain chip and the optical modulatorwithin the optimal operating temperature range of the optical modulatorat which an operating current of the gain chip is minimized.
 3. Theexternal cavity optical transmitter as defined in claim 2, wherein thetemperature controller varies the temperature of the optical modulatorand the gain chip to determine a temperature at which the operatingcurrent of the gain chip is minimized.
 4. The external cavity opticaltransmitter as defined in claim 3, further including an etalon adjacentthe reflector.
 5. An external cavity optical transmitter comprising: atemperature controlled substrate; a gain chip having a reflectiveportion, wherein the gain chip is disposed on the temperature controlledsubstrate and is adapted to emit optical energy; a grating disposed onthe temperature controlled substrate to receive optical energy emittedby the gain chip and to reflect at least a portion of the optical energyemitted by the gain chip; a reflector disposed on the temperaturecontrolled substrate to receive optical energy reflected from thegrating, the reflector and the reflective portion of the gain chipforming an optical resonant structure; an optical modulator to modulateoptical energy from the gain chip, wherein the optical modulator has anoptimal operating temperature range; and a temperature controllercoupled to the temperature controlled substrate to maintain thetemperature controlled substrate and the gain chip at a temperaturewithin the optimal operating temperature range of the optical modulator.6. The external cavity optical transmitter as defined in claim 5,wherein the temperature controller maintains the temperature controlledsubstrate at a temperature within the optimal operating temperaturerange of the optical modulator at which an operating current of the gainchip is minimized.
 7. The external cavity optical transmitter as definedin claim 6, wherein the temperature controller varies the temperature ofthe temperature controlled substrate within the optimal operatingtemperature range of the optical modulator to determine a temperature ofthe temperature controlled substrate at which the operating current ofthe gain chip is minimized.
 8. The external cavity optical transmitteras defined in claim 5, further comprising: a variable optical phaseretardation device disposed between the gain chip and the reflector,wherein the variable optical phase retardation device retards a phase ofoptical energy passing between the gain chip and the reflector byvarying a refractive index of the variable optical phase retardationdevice; and a voltage source coupled to the variable optical phaseretardation device and controlled to maintain the refractive index ofthe variable optical phase retardation device at a level at which theoperating current of the gain chip is minimized.
 9. The external cavityoptical transmitter as defined in claim 8, wherein the voltage source iscontrolled to vary the refractive index of the variable optical phaseretardation device to determine a phase retardation at which theoperating current of the gain chip is minimized.
 10. The external cavityoptical transmitter as defined in claim 5, wherein the temperaturecontroller varies the temperature of the temperature controlledsubstrate over a range of temperatures to determine a temperature atwhich the operating current of the gain chip is minimized. 11.(canceled)
 12. (canceled)
 13. (canceled)
 14. (canceled)
 15. (canceled)16. (canceled)
 17. A method of operating an external cavity opticaltransmitter including a gain chip, an optical modulator and anelectro-optic crystal coupled to a temperature controlled substratehaving an associated temperature, the method comprising: maintaining thetemperature controlled substrate, the gain chip and the opticalmodulator between a first temperature and a second temperature betweenwhich the optical modulator has a predefined performance characteristic;varying a bias voltage applied to the electro-optic crystal from a firstvoltage to a second voltage; measuring an operating current supplied tothe gain chip as the bias voltage applied to the electro-optic crystalvaries from the first voltage to the second voltage; determining anoptimum bias voltage between the first voltage and the second voltagethat corresponds to a minimum operating current; and maintaining thebias voltage applied to the electro-optic crystal at the optimum biasvoltage.
 18. The method as defined in claim 17, wherein determining anoptimum bias voltage comprises determining operating current as afunction of bias voltage.
 19. The method as defined in claim 18, whereindetermining the optimum bias voltage comprises taking a derivative ofthe operating current as a function of bias voltage.
 20. The method asdefined in claim 18, wherein determining the optimum bias voltagecomprises taking first and second derivatives of the operating currentas a function of bias voltage.
 21. The method as defined in claim 17,further comprising: measuring the operating current and comparing it tothe minimum operating current; and if a difference between the operatingcurrent and the minimum operating current becomes significant, (a)varying the bias voltage applied to the electro-optic crystal from thefirst voltage to a second voltage, (b) measuring the operating currentsupplied to the gain chip as the bias voltage applied to theelectro-optic crystal varies from the first voltage to the secondvoltage, (c) determining a second optimum bias voltage that correspondsto a second minimum operating current, and (d) maintaining the biasvoltage applied to the electro-optic crystal at the second optimum biasvoltage.
 22. A method of manufacturing an external cavity opticaltransmitter, the method comprising: selecting an optical modulatorhaving an optimal operating temperature range defined by a firsttemperature and a second temperature; placing a gain chip, a grating anda mirror on a substrate that will be maintained at a temperature betweenthe first temperature and the second temperature; placing a lens mountedto a bipod flexture onto the substrate in proximity to the gain chip;varying the temperature of the substrate between the first temperatureand the second temperature to determine a temperature corresponding to aminimum current provided to the gain chip; and placing the opticalmodulator on the substrate.
 23. The method as defined in claim 22,wherein the optimal operating temperature range is defined by chirpperformance of the optical modulator.
 24. The method as defined in claim22, further comprising tuning a gain chip lasing wavelength by placingthe lens in an optimized position.
 25. The method as defined in claim22, further comprising placing the substrate on a temperature controlledplatform.
 26. The method as defined in claim 22, further comprisinghermetically packaging the optical transmitter.